It is advantageous to be able to measure two or more signals virtually simultaneously. A battery measurement system can be used to determine the capacity of a battery at a point in time. It requires the measurement of the battery terminal voltage, the battery current drawn and, optionally, the battery temperature, and subsequent processing of these results.
However, to accurately determine the remaining charge in the battery and the battery's ability to deliver that charge it is required that the battery voltage and battery current must be measured so that the measured data reflects the state of these parameters with a minimum timing delay between them. This measured data is processed by algorithms running on a digital processor that implement the calculations required to determine the capacity of the battery. This is particularly the case in an automotive battery measurement system, where the ability of the battery to deliver charge can be determined from simultaneously measuring these battery parameters when large currents are being drawn, for example when the engine is being started.
In a typical analog-to-digital converter (ADC), anti-aliasing (AA) filters are generally required before the ADC, to limit the aliasing of signals at frequencies greater than one-half of the ADC sampling rate into the signal bandwidth. These anti-aliasing filters introduce delays in the signal path, affecting the synchronicity of the measured signals, (signals presented to the ADC) and they generally add cost and area. The anti-aliasing filters are generally combined with other circuitry to form signal-conditioning circuits. These signal-conditioning circuits have to perform other tasks in addition to providing anti-alias filtering, for example, attenuation or amplification of the input signals in a battery measurement system.
In a battery measurement system, signals representing the battery voltage and current are applied to one or more ADCs, and the results of the ADC conversions generally undergo some processing in a digital processor to determine the capacity of the battery. The digital processor typically also performs other tasks, such as communicating information about the state of the battery to a host processor over a communications interface, or storing parameter data into non-volatile memory. If the processor is busy servicing another task, such as a communication protocol, it is possible that ADC conversions may be missed because the processor is not available to process this data. This could result in the loss of information about the state of the battery, for example if this occurs during a period of large current flow when the synchronized voltage and current data is required.
In a battery measurement system there may be periods of time when the measurement system is required to enter a state of lower power consumption. In this state it may be required to continue to perform a certain level of measurement of battery performance, but possibly of only one battery parameter and with limited processing of this data. After a particular set of measurement criteria has been met, it maybe required to automatically exit from the lower power consumption state and then recommence the synchronous voltage and current conversions and subsequent calculations required of the algorithm to determine the capacity of the battery.
In a battery measurement system it is generally required to measure the temperature of the battery or its local environment. This measurement could be either performed by a dedicated ADC or by multiplexing the signal from a temperature transducer into one of the ADCs present for measuring voltage or current, in which case this ADC must return to be synchronous with the other ADC when the temperature measurement is complete.